The basic problem comes about by using the same battery or batteries for engine starting and for powering the chartplotter
(and, by extension, other devices on board). Starters
draw a lot of current and will pull the voltage down to levels which may be too low for certain devices, like your chartplotter
While larger wires may help a bit, they won't solve the basic problem.
The real solution is to separate the house batteries from the start battery. With only two batteries onboard, that's difficult. Ideally, you'd use those two together as the house bank, and add a start battery dedicated to the engine. But, if that's not possible, then you will want to always start the engine with both batteries together, use larger wiring, and hope that will do the trick.
I'm NOT a fan of using house batteries for engine starting, for several reasons which have been hashed out over and over on this and other Boards. Some people do it and get away with it, but it's just not good practice. Much better to separate the start circuit from the house circuit.